Grazing impacts on the spatial distribution of soil microbial biomass around tussock grasses in a tropical grassland

The role of grass tussocks in supporting soil microbial biomass (SMB) in grazed ecosystems is not fully understood, nor is the spatial distribution of SMB in response to different grass species. We undertook a study in 1997 that examined fine-scale distributions of SMB in grazed experimental paddocks located in the eucalypt woodlands of northern Queensland, Australia. Levels of SMB were determined on soil samples collected at seven locations along 60 cm transects in the vicinity of three species of tussock grass (Bothriochloa ewartiana, Chrysopogon fallax, Heteropogon contortus) and bare spaces, in replicate paddocks under five different grazing regimes. Data (n = 280) were analysed as a split-split plot in a randomised complete block. Slope position (block) and paddock management were the main plots, micro-patch type (tussocks and bare spaces) the split plot, and location around micro-patch the split-split plot. Paddock management, micro-patch, and location effects were significantly (p < 0.05) different, as were management x location and micro-patch x location interactions. The highest SMB levels were recorded at tussock centres on ungrazed (control) and lightly grazed paddocks, with lower levels recorded on degrading (15-27% reduction), and degraded/recovering (40-53% reduction) sites. Successively lower levels were noted from tussock centres outwards to the most distant locations (+30, -30 cm) with level of paddock degradation. High levels of SMB were noted around tussocks of B. ewartiana and C. fallax, while the lowest were recorded across bare patches (59% of levels for the above species). Heavy grazing reduced inputs of organic materials and carbon into the soil, thereby limiting resources available for microbial growth, Fine-scale monitoring of the plant-microbe-soil interface should be combined with large-scale measures of landscape response to properly describe degradation and recovery processes. (C) 1999 Elsevier Science B.V. All rights reserved.